Fix color space issue for gaussian splats.

[dli7319]

By default, the simulator renders the main scene to an render target, then composites that render target over the simulator scene, isolating the main scene's blending from the simulator scene just like actual XR rendering.

When rendering to a render target, three.js sets the output color space of shaders to linear using a linearToOutputTexel function within each shader. However, sparkjs does not include this function inside it's gaussian splatting shader so it continues to output SRGB unless we request it to output linear.

Note that technically the simulator will still look slightly different than on device because alpha blending in linear will be different than alpha blending in SRGB.

[nsalminen]

Thanks so much @dli7319!